The present invention relates to a fluid pressure control device for a vehicle braking system, and more particularly to a fluid pressure control device of the type in which an inertia-controlled valve unit is adapted to control the braking pressure applied to rear wheel brake cylinders from a master cylinder in accordance with the rate of deceleration of the vehicle.
An inertia-controlled valve unit of the type includes a movable ball disposed within a valve chamber to co-operate with a valve seat and rolling towards the valve seat on the inclined bottom of the chamber to cut-off excessive braking pressure applied to the rear wheel cylinders. The valve unit is usually mounted at an inclined angle on a vehicle body frame supported by a suspension system such that the ball rests under gravity in a normal position to open the valve and rolls towards the valve seat to close the valve when the ball is subjected to a deceleration is excess of a predetermined value. In practical use of the valve unit, if the master cylinder is rapidly operated by emergency depression of the brake pedal, the emergency braking operation causes forward displacement of the vehicle body frame against the spring suspended wheel axles due to the moment of inertia. This decreases the rate of deceleration acting on the ball less than that acting on the wheel axles and subsequently the rolling operation of the ball delays to occur mis-operation of the valve unit.